xa
xa is a high-speed, two-pass portable cross-assembler. It understands mnemonics and generates code for NMOS 6502s (such as 6502A, 6504, 6507, 6510, 7501, 8500, 8501, 8502 …), CMOS 6502s (65C02 and Rockwell R65C02) and the 65816.
This looks to be packaged on Ubuntu as xa65.